Inert gases such as nitrogen, argon, helium, and the like are widely employed by industries to protect materials from exposure to oxidizing environment. For example, inert gases such as argon, nitrogen, and helium are commonly used today to shield materials during welding, spraying metallic and ceramic materials by thermal and plasma techniques, depositing coatings by chemical vapor and physical vapor deposition techniques, and melting and refining ferrous and non-ferrous metals and alloys. They are also used to provide inert atmosphere for processing composites, semiconductor materials, and chemicals, packaging electronics and food products, removing dissolved gases from chemicals, fruit juices and edible oils, vulcanizing rubber and curing tires, and heat treating ferrous and non-ferrous metals and alloys, ceramics, composites, and metal matrix materials. Inert gases used in these applications are required to be as pure as possible. They are also required to be substantially free of oxygen because the presence of oxygen as an impurity results in oxidation of the processed materials.
A major portion of nitrogen used in above applications has been produced by distillation of air in large cryogenic plants. The cryogenically produced nitrogen is generally very pure (contains less than 10 ppm by volume residual oxygen) and expensive. To reduce the cost of nitrogen, several non-cryogenic air separation techniques such as adsorption and permeation have been recently developed and introduced in the market. The non-cryogenically produced nitrogen is much less expensive, but it contains a considerably higher level of residual oxygen (0.1 to 5.4 by volume) than that produced cryogenically, making a direct substitution of cryogenically produced nitrogen with non-cryogenically produced nitrogen in processing oxygen sensitive materials very difficult if not impossible.
Several processes have been developed and used commercially today to remove oxygen from inert gases prior to using them in processing oxygensensitive materials. For example, Cu/CuO and Ni/NiO based catalysts have been developed and used extensively to chemically scavenge residual oxygen from inert gases. These systems generally remove oxygen by absorbing it on metals highly dispersed on an inert support. They require frequent regeneration at high temperature with hydrogen, and are used in cyclic fashion, i.e. absorption followed by regeneration. These catalyst systems are described in detail in BASF Technical Leaflet on BASF-Catalyst R3-11 and U.S. Pat. No. 4,713,224. Since the oxygen absorption capacity of these catalyst systems is limited, they are generally used to purify inert gases containing less than 1,000 ppm or 0.14 by volume residual oxygen. They are not economically attractive to remove residual oxygen from nitrogen streams generated by non-cryogenic air separation techniques.
Another process that has been developed and commercially used today by industries involves converting residual oxygen to moisture with expensive hydrogen over a platinum group metal catalyst. It requires use of more than a stoichiometric amount of hydrogen for converting residual oxygen to moisture. The treated inert gas stream is optionally processed further to remove moisture, thereby producing dry, oxygen-free inert gas stream. This catalytic process is disclosed in U.S. Pat. Nos. 3,535,074, 4,931,070, 5,004,482 and 5,004,489. Since hydrogen required for converting oxygen to moisture is expensive, this process is generally used to purify inert gases containing up to 1,0001ppm or 0.1% by volume residual oxygen. It can, however, be used to purify non-cryogenically produced nitrogen in countries where hydrogen is readily available at low cost.
U.S. Pat. No. 3,535,074 discloses a process for removing residual oxygen by reacting it with hydrogen over a platinum group metal catalyst followed by absorption of unreacted oxygen with Cu/CuO or Ni/NiO based catalyst. This process is most suitable for producing inert gases, free of both residual oxygen and unconverted hydrogen, generally required for processing semiconductor materials. It is an expensive process, and is generally used to purify inert gases containing less than 1,000PPM or 0.1% by volume residual oxygen. It is not economical to use this process for purifying non-cryogenically produced nitrogen. n
U.S. Pat. No. 4,859,435 discloses a process for removing minor amounts of oxygen from inert gas streams to result in very low levels of oxygen contamination in such inert gas streams. According to the Patent, residual oxygen is removed by reacting it with stoichiometric amount of methanol over a platinum group metal catalyst. The treated inert gas stream is optionally processed further to remove moisture and carbon dioxide, thereby producing dry, carbon dioxide-free and oxygen-free inert gas stream. This process is most suitable for removing oxygen from non-cryogenically produced nitrogen. However, it has not been used in many parts of the world because of the costs involved in installing methanol delivery system.
Based upon the above discussion, it is clear that there is a need to develop a process for removing residual oxygen inexpensively from non-cryogenically produced nitrogen. Additionally, there is a need to develop a process which eliminates need of both expensive hydrogen and an auxiliary system for delivering methanol.